1. Field of the Invention
The present invention relates to a coating solution for forming a silica coating and a method of forming such a silica coating, and more particularly to a coating solution for forming a silica coating which is free from cracks even if it has a large thickness, is highly homogeneous and dense, and provides a highly flat surface even on a substrate having surface irregularities or steps, and a method of forming such a silica coating.
2. Description of the Related Art
Recent rapid development of the VLSI fabrication technology imposes stricter requirements for more stacked layers, higher operation speeds, and more functions in the multilayer interconnection technology. Specifically, the fabrication of semiconductor devices requires interconnection patterns and insulating films to be formed on substrates. Such interconnection patterns and insulating films on substrates necessarily produce surface steps or irregularities. If an interconnection pattern is to be formed directly on such surface steps or irregularities, then the interconnection pattern cannot be defined accurately to fine details due to those surface steps or irregularities. For this reason, it has been customary to flatten the substrate surface to remove any undesired steps or irregularities therefrom.
Heretofore, the spin-on-glass (SOG) coating process is generally employed to make substrate surfaces even to remove surface steps or irregularities therefrom. According to the SOG coating process, a coating solution is prepared which comprises tetraalkoxysilane dissolved in an organic solvent mainly of alcohol, and applied to a substrate surface having steps. After the coating solution is coated to fill recesses in the substrate surface and cover the entire substrate surface, the assembly is heated to produce a silica coating, thereby flattening the substrate surface.
Since the above SOG coating process employs tetraalkoxysilane, the produced silica coating is a completely inorganic coating and has highly reliable properties. However, the coating solution has such a low viscosity that it will produce a silica coating having a thickness up to a maximum of 0.4 .mu.m in one coating cycle. Such a thickness is not large enough to eliminate surface irregularities on the substrate in one coating process. To achieve a desired coating thickness, the coating process has to be repeated a number of times. However, the coating produced by repeating the coating process is susceptible to cracks and hence is not acceptable for practical use.
The above shortcomings may be eliminated by a coating solution for forming a silicon coating of alkoxysilane with an alkyl group partly introduced therein, as disclosed in Japanese laid-open patent publication No. 63-221076. The disclosed coating solution comprises a combination of tetraalkoxysilane and monoalkyltrialkoxysilane or dialkylalkoxysilane. Since an organic group remains after hydrolysis, the coating solution has an excessive viscosity, allowing a coating having a thickness of 1 .mu.m or greater to be formed when it is applied in one coating cycle. However, the coating produced by the disclosed coating solution is less flat than the coating produced by the coating solution of tetraalkoxysilane, and tends to produce wrinkle-like irregularities on the surface of the coating.
Various improved coating apparatus have been proposed in recent years to increase the flatness of silica coating surfaces. For example, rotary cup coating apparatus has been developed for use in place of rotary coating apparatus known as a spinner. The rotary cup coating apparatus are effective to produce flatter coatings on substrates than the conventional spinners. Nevertheless, the existing rotary cup coating apparatus are not effective enough to produce coatings that are completely free from wrinkle-like irregularities on their surfaces.
Based on a material-oriented approach, there have been demands in the art for a coating solution for forming a silica coating with better surface flatness, particularly a coating solution which can produce a silica coating with a highly flat surface, in combination with rotary cup coating apparatus.